The application relates to guidewires configured for intraluminal application in medical procedures, and methods of their manufacture. More specifically, the application relates to guidewires that possess varying properties of flexibility and torsional stiffness along their length, and methods for making them.
Guidewires have long been known and used in the art of minimally invasive medical practice. Guidewires are typically used in conjunction with catheters in a procedure under which a placement catheter may first be threaded into the vasculature of a patient to a desired location using known techniques. A lumen within the placement catheter permits the physician to insert a guidewire through the catheter to the same location. Thereafter, when the physician may need to sequentially place a second, or third, or even a fourth catheter to the same location, it is a simple matter to withdraw the catheter while leaving the guidewire in place. After this action, second, third, and fourth etc. catheters may be sequentially introduced and withdrawn over the guidewire that was left in place. In other techniques, a guidewire may be introduced into the vasculature of a patient without the assistance of a placement catheter, and once in position, catheters may be sequentially inserted over the guidewire as desired.
It is typical that best medical practice for anatomical insertion requires a guidewire that has behavioral characteristics that vary along its length. For example, under some conditions, the distal end of the guidewire may be required to be more flexible than the proximal end so that the distal end may more easily be threaded around the more tortuous distal branches of the luminal anatomy. Further, the proximal end of the guidewire may be required to have greater torsional stiffness than the distal end because, upon rotation of the guidewire, the proximal end must carry all the torsional forces that are transmitted down the length of the guidewire, including what is required to overcome cumulative frictional losses.
Finally, the distal end of a guidewire should be selectively formable, so that the treating physician may apply a curve to the tip of the guidewire in order to facilitate navigation along the tortuous passageways of the vascular anatomy. By selectively formable, it is meant that the wire from which guidewire core is made may be bent to a particular shape and that the shape will be maintained by the wire. This allows the physician to impart a particular shape to the guidewire, by bending or kinking it for example, to facilitate steering its placement into a patient's vasculature. To provide this selective formability, in typical embodiments, the entire core wire may be made of stainless steel. However, other materials may be used to provide this feature. The use of a formable material, such as stainless steel, provides advantages in the guide wire over materials that cannot be formed, such as superelastic materials like Nitinol. Superelastic materials like Nitinol are so resilient that they tend to spring back to their original shape even if bent, thus are not formable. Although superelastic material may be provided with a “preformed” memory shape, such a preformed shape is typically determined in the manufacture of the guide wire and cannot readily be altered or modified by the physician by simply bending the guide wire prior to use. Although use of superelastic materials such as Nitinol in guide wire applications may provide some advantages in certain uses, a formable core, such as of stainless steel, which can be formed by the physician to a shape suitable for a particular patient or preferred by that physician, provides an advantage that cannot be obtained with a superelastic core guide wire.
Thus, certain solutions have been developed in the prior art to address these requirements. In one typical solution, a guidewire may be fabricated by applying the same metallurgical process along the entire length of an initial ingot of uniform metallurgical properties and uniform diameter that will be converted into the guidewire. The initial ingot may be taken up and cold worked along its entire length, or annealed, or swaged, or whatever process is required to impart the desired characteristics to the metal of the final guidewire product. Once these metallurgical processes have been performed on the wire as a whole, the wire obtained from the worked ingot may be geometrically shaped in order to impart desired different flexibilities, torsional stiffnesses and the like that are desired in the final guidewire product. For example, the wire obtained from a worked ingot may be shaped by known process such as chemical washes, polishes, grinding, or compressing, to have a distal end with a diameter that is smaller than the diameter of the proximal end. By this means, the distal end will be given greater flexibility but less torsional resistance than the proximal end. A shaped guidewire 10 of the kind described is depicted in FIG. 1 where it may be seen that a core metal element 12 having a configuration with varying diameter sizes along its length is coated in a polymer 14, or other suitable material. The coating may be configured to impart a more uniform outside diameter to the overall guidewire 10. Alternatively, one or more wire coils may be used instead of or in conjunction with a polymer coating for similar purpose.
In another typical solution, different pieces of wire may be formed by different processes to have different properties. These pieces of wire may then be joined or connected together into a single guidewire core using known jointing processes, to provide a resulting guidewire with varying properties along its length. For example, as may be envisaged with reference to FIG. 5 through FIG. 9, different embodiments 20a, 20b, and 20c show how a superelastic portion of wire 22a, 22b, and 22c made from Nitinol or similar metal, may be joined to a portion of wire 24a, 24b, and 24c that has linear elastic properties using joining methods such as welding, soldering, brazing, or covering with a jacket 26b, or inserting a filler 28c. These types of joints between portions of a wire having different metallurgical properties are referred to herein as “mechanical” joints. These mechanical joints are to be distinguished from interfaces (that will be described in the invention below) between different portions of a single unitary wire which have different metallurgical properties arising from having different metallurgical processes applied to those portions while still part of the single unitary wire.
Thus, in a core wire having this combination of a distinct and mechanically joined formable distal portion and a superelastic proximal portion, desired shapes may be imparted by a physician to the distal end of the guide wire to facilitate making turns, etc., in tortuous vessel passages, while in the same guide wire the more proximal portion would possess superelastic properties to allow it to follow the distal portion through the tortuous passages without permanently deforming.
However, problems may arise in the prior art as described. Welds and solder or braze joints are generally undesirable on a guidewire because they introduce a potential point of kinking or fracture. Furthermore, discrete steps in the gradient of a guidewire diameter that are introduced by grinding or other known means may also introduce potential points at which stress is raised to produce cracking or fracture.
Thus there is a need in the art for a system and method for a guidewire that solves the problems in the prior art. The present invention addresses these and other needs.